1. Field
Embodiments of the invention relate to an electronic throttle control system and more particularly to an electronic throttle control system for controlling the angle (position) of a throttle valve by motor drive.
2. Description of the Related Art
In motorcycles and passenger cars, an electronic throttle control system is used that is based on an application of a throttle-by-wire (TBW) control, whereby an operating amount of an accelerator (e.g., grip or pedal) is detected. An optimum angle of a throttle valve is calculated based on the detected accelerator angle and signals from various sensors. A motor is driven based on the calculated target angle to open or close the throttle valve.
Japanese Patent Publication No. 2008-088925 (“JP 2008-088925”) discloses an electronic throttle control system in which a lower limit value, greater than a full closure angle (i.e., full closure position) of a throttle valve by a predetermined angle, is set. In this control system, the lower limit value is updated until an opener lever connected to a throttle shaft for turning the throttle valve abuts a full closing stopper. The angle at the moment of the abutment is set as the lower limit value to reduce an idle speed.
In the control system disclosed in JP 2008-088925, it is necessary to abut the opener lever against the full closing stopper to maintain the idle speed at an appropriate level, and therefore, a heavy load is exerted on a reduction gear for driving the throttle valve. On the other hand, to control the throttle angle without using such a stopper, it is necessary to perform a control using a limit value, such that the throttle valve will not interfere with an intake passage under presumed operating conditions. However, in an engine required to exhibit a high output relative to engine displacement, the diameter of the intake passage is set at a high value (i.e., overbore), and dispersions of devices and sensors and overshoots of control are generated. Accordingly, it has been difficult to set a limit value which enables an appropriate setting of an idle speed.